The present invention concerns a process and system for the extrusion of a foamed polymer sheet into a cooling atmosphere and over a diameter expanding sizing drum. More specifically, this invention concerns a process and system for the formation of tubular polymer foamed sheet of high dimensional uniformity and of at least substantially uniform orientation.
In U.S. Pat. Nos. 3,864,444 and 4,352,893 there is disclosed processes and systems for the extrusion of tubular polystyrene into a water bath, which water bath may either be boiling or at a temperature below the boiling point of water. After passing through the water bath, the tubular extruded polystyrene passes over a diameter expanding sizing drum which is designed to give the extruded tube a uniform internal diameter. While this system is described primarily for the vertical extrusion of polystyrene and the passage of the tubular extrudate vertically up through a bath of water, it has been found that the process and apparatus can be advantageously employed for horizontal extrusion of the tubular polystyrene foam sheet.
Polystyrene foam sheet also is conventionally extruded into air as the cooling atmosphere between the die and the sizing drum.
When employing the system for horizontal extrusion into a water bath, the water is usually employed externally of the polystyrene foam tube. In so processing horizontally extruded polystyrene, a problem was observed downstream during the thermoformation of the foamed polystyrene into desired parts. After the tubular foam polystyrene sheet is formed it is slit open to form a flat sheet. Following an appropriate aging technique, the polystyrene foam sheet is ready for thermoformation into any desired product, for example, egg cartons, sandwich containers, coffee cups, etc. Prior to the actual thermoformation step, the foam sheet is preheated and during this preheating step, buckling and sagging has been observed across the width of the sheet. It has been determined that this problem is caused by a non-uniformity in orientation in the foam sheet in combination with slight variations in cross-sectional dimension. Analysis of the problem has resulted in a determination that the cause stems from the difference in water pressure in the water bath as the extruded tube passes therethrough. Because the water pressure is greater at the bottom of the water bath than at the top, the extruded tube, which at this point contains an air bubble, is distorted because of the difference in pressure. Thus, instead of the extruded polymer being the projection of an expanding perfect circle, it is the projection of an imperfect circle. This imperfection becomes set in the extruded polymer foam sheet and the result is a sheet which has nonuniform biaxial orientation and can in some instances contain unwanted dimensional differences.
It is an object of this invention to present a process for the production of polymer foam sheet extrudate having uniform dimensions and uniform orientation.
A further object of this invention is to present a system for the production of a foam sheet extrudate having uniform biaxial orientation and dimensions.
Yet another object of this invention is to present a process and system for the gas or liquid cooled extrusion of polymeric foam sheet.